1. Field of the Invention
This invention relates generally to an infrared intrusion sensing system and in particular to a method and apparatus for restricting superfluous radiation not focused on a detector.
2. Description of Related Art
An infrared intrusion sensing system comprises a lens having a plurality of lines of focus for focusing infrared radiation that enters the system onto a detector. An enclosure surrounding the detector provides isolation from insect entry. The enclosure envelope is a protective volume of space for the optical path between the lens and the detector. Radiation focused by the lens in locations other than the detector is "superfluous" and is considered a source of false intrusion sensing by internal re-reflection inside the insect exclusion envelope. Elimination of superfluous radiation reaching the detector results in improved performance of the infrared intrusion sensing system and minimizes false alarms.
U.S. Pat. No. 4,268,347 issued May 19, 1981 to Richard B. Stephens describes a process for forming reflectivity surfaces by particle track etching of a dielectric material. A textured surface is produced having conical cavities. The textured surface reduces light reflections, but it does not describe or suggest application in the infrared regions.
U.S. Pat. No. 4,271,358 issued Jun. 2, 1981 to Frank Schwarz describes the use of polyhydrocarbon plastics such as polyethylene, for the housing of infrared sensors and adding dyes or coloring water which strongly absorb in the visible but which are not strong absorbers in the farther infrared.
U.S. Pat. No. 5,424,718 issued Jan. 13, 1995 to Kurt Muhler et al. describes an IR intrusion detector using scattering to prevent false alarms by radiation outside the useful radiation band. Focusing mirrors are provided with a rough surface for infrared selectivity. In the wavelength range from 6 to 15 micrometers, the infrared radiation is specularly reflected and focused in accordance with the shape of the mirrors. Extraneous radiation in the visible and near-infrared range from about 0.4 micrometer or less up to 3 micrometers is diffusely scattered. However, there is no scattering of extraneous infrared radiation to prevent such radiation from being detected.